1. FIELD OF THE INVENTION
This invention relates to catalytic combustion apparatus, and more particularly to a low NOx catalytic combustion apparatus having a cylindrical housing defining a fluid passage, and a plurality of combustion catalyst bodies arranged face to face in a direction of fluid flow through the cylindrical housing and defining numerous bores extending in the direction of fluid flow.
2. DESCRIPTION OF THE RELATED ART
In view of the increasing environmental pollution by NOx discharged from combustion equipment, there is a desire for means to achieve a drastic reduction in NOx produced in combustion processes. One of such NOx reducing means is a premixed catalytic combustion method which uses combustion catalysts in honeycomb form having numerous bores extending in a direction of fluid flow. This method is known to assure stable combustion at 1100.degree. to 1300.degree. C. while drastically reducing generation of NOx. Studies are being made on the possibility of utilizing this feature as means to realize an extremely low NOx generation in gas turbines. Studies are also made as to its use as means to afterburn an exhaust fuel and air mixture from fuel cells. Further, studies are made as to application thereof to boilers and burners used in industry.
A combustion catalyst in plate form has been developed heretofore, which includes a rare metal such as palladium or platinum supported by a cordierite honeycomb base through a coating material such as alumina. The cordierite honeycomb base is a material that achieves a considerably low coefficient of thermal expansion in the order of 1.4.times.10.sup.-6 /.degree.C. However, this honeycomb base is said to have a maximum working temperature at 1400.degree. C., and thus has its limitations in use at high temperatures. The catalyst having this construction encounters a deterioration in activity at temperatures above 1000.degree. C. This is due to a reduction in specific surface area caused by vaporization of the rare metal and sintering of the coating material.
Under the circumstances, Inventors have proposed a catalytic combustion apparatus employing a palladium/cordierite combustion catalyst in a low temperature, upstream stage, and a manganese substituted laminar aluminate catalyst in a high temperature, intermediate to downstream stage. The manganese substituted laminar aluminate catalyst has a melting point at 1600.degree. C. or above. Thus, this catalyst has a characteristic to remain highly active even at 1300.degree. C., with a large specific surface area maintained over a long period of time.
In the conventional construction, each catalyst body is formed into a plate covering an entire sectional area perpendicular to a direction of fluid flow, with peripheral edges thereof bonded to a cylindrical housing. A plurality of such catalyst bodies are arranged at intervals in the direction of fluid flow. These intervals serve to limit thickness of each catalyst body, and to diminish temperature variations in the direction of its thickness. thereby to suppress thermal stress. Further, the intervals are effective to hamper an increase in resistance to gas flows due to displacement or non-alignment among the bores formed in the catalyst bodies.
However, where the combustion catalyst bodies are fixed to the cylindrical housing with the peripheral edges bonded thereto, the catalyst bodies cannot expand or contract freely with temperature variations. Thus, the catalyst bodies could be damaged by the thermal stress resulting therefrom.
An attempt as shown in FIG. 14 has been made to solve the above problem. The illustrated apparatus includes combustion catalyst bodies 5 having peripheral edges just opposed to a cylindrical housing 3 without being bonded thereto. An annular metallic spacer 9, for example, is placed between an adjacent pair of catalyst bodies 5 to secure a spacing therebetween for releasing thermal stress. With this construction, however, a fluid pressure applied to an upstream catalyst body 5 is passed on to a next catalyst body 5 through the spacer 9 therebetween. This occurs successively from upstream to downstream until the final catalyst body 5 is subjected to a great concentration of forces which could damage this catalyst body 5. Furthermore, temperature differences tend to occur between portions of the catalyst bodies 5 contacting the metallic spacers 9 and portions out of contact with the spacers 9. Such temperature differences could promote damage to the catalyst bodies 5.
Further, an increased combustion capacity is needed in order that the above high temperature combustion catalyst may be better suited for practical purposes. To increase the quantity of gas processing by the combustion catalyst bodies 5 without impairing their ability to lower NOx level, the catalyst bodies must have increased areas. In this case, however, there is an inevitable limitation of size in forming an integral honeycomb structure while maintaining dimensional precision and strength. For example, a high degree of activity is required in addition to strength. Where a combustion catalyst body is required to have at least 200 cells per square inch, a size about 200 mm in diameter or 200 mm square is considered its limitation.
Studies made heretofore to achieve an increased capacity include, for example, a method in which small catalytic combustion apparatus are manufactured and connected in parallel to form a multiple type assembly, and a method in which a plurality of honeycomb catalyst segments having a size not exceeding a size corresponding to the 200 mm diameter are joined or bonded together to increase sectional areas.
However, the former multiple type assembly of catalytic combustion apparatus connected in parallel has a large and complicated overall construction which is expensive to manufacture and difficult to maintain. Thus, such an assembly is hardly practicable.
The latter bonding method uses a material different from the combustion catalyst for bonding purposes. Since the combustion catalyst is used at temperatures exceeding 1000.degree. C., a solid phase reaction between the two different materials tends to cause a deterioration in the joints. The catalyst segments may be bonded by using a similar material, with the advantages that the solid phase reaction between the materials may be suppressed and that the materials have the same coefficient of thermal expansion. However, it is difficult to form joints having a thickness corresponding to that of cell walls. The longer the bonding surfaces are, the less uniform become mechanical strength and temperature distribution due to the unevenness in thickness. This gives rise to the problem that cracks tend to be formed in regions adjacent the joints. This problem is serious particularly with the manganese substituted laminar aluminate catalyst which has a greater coefficient of thermal expansion, 6 to 8.times.10.sup.-6 /.degree.C., than the cordierite honeycomb combustion catalyst, and which is subjected to a thermal stress several times that of the latter catalyst.
The honeycomb catalyst could be damaged also by a stress concentration due to a difference in coefficient of thermal expansion, heat conduction or mechanical strength between the metallic spacer and honeycomb catalyst.